The present invention relates to an intrusion alarm system provided with a passive sensor with a detector for detecting light energy (electromagnetic radiation) from an object in a location to be monitored, and with an alarm for generating an alarm signal, dependent on whether a detection signal is emitted by the detector or not.
Such an intrusion alarm system is known from European patent application No. 0 255 812 in the name of Elkron S.p.A. The intrusion alarm system described therein utilizes, in a well-known manner, a passive infrared sensor, whereby infrared light emitted by an object in a location to be monitored is passed by optical means--via an entrance of a passive infrared sensor--to a detector in the form of, for example, a pyro-electric element. The optical means can, for example, consist of a mirror or a Fresnel lens. An intruder in the location to be monitored is spotted as a result of the pyro-electric element detecting a change, generated by the intruder, in the amount of infrared light falling thereon and consequently activating an alarm, which alarm generates an alarm signal. In order to optimize the operation of the known intrusion alarm system the aforesaid European patent application proposes to couple the passive infrared sensor to a radio frequency sensor operating in the UHF band. With the known intrusion alarm system an alarm signal is not initiated unless both the passive infrared sensor and the radio frequency sensor, independently, detect an intruder in the location to be monitored. While the detection operation of the passive infrared sensor is already outlined above, the detection operation of the radio frequency sensor is in broad outline as follows. Movements made by an intruder in a location to be monitored, in which radio waves are emitted, cause a disturbance of the radio frequency band (as a result of the Doppler effect), which disturbance is detected by the radio frequency sensor, resulting in an alarm signal being generated.
Such an intrusion alarm system is also known from Berman U.S. Pat. No. 3,703,718. The infrared intrusion alarm system described therein utilizes a single passive sensor and optical means for focusing radiation directed at the passive sensor from various fields of vision in a location to be monitored. An amplifier, which is arranged so as to have a frequency response corresponding with the walking speed of an intruder, amplifies the signal from the passive sensor. The amplifier is provided with means for distinguishing between changes in the infrared radiation that are caused by the presence of an intruder and changes caused by gradual temperature changes, such as changes in the room temperature and the ambient temperature.
One drawback of the known intrusion alarm system is that it does not offer a solution for the following problem. Since the operation of the passive infrared sensor is based on the detection of infrared light, i.e. heat radiation with a wavelength in the order of in particular approximately 6-18 .mu.m, emitted by an intruder in a location to be monitored, and since only very few materials possess good transmission characteristics for such infrared light (nearly all materials block, absorb and/or reflect this kind of light), the detection of the known intrusion alarm system can be easily sabotaged by placing materials that possess poor transmission characteristics for this kind of infrared light on and/or near the detector of the passive infrared sensor. When, for example, at least part of the receptor of the passive infrared sensor is blocked with materials such as paper, glass, paint, cardboard or plastic, the monitoring ability of the known intrusion alarm system is very detrimental. In some cases such sabotaging of the quality of the known intrusion alarm system can be carried out without this being clearly visible to the user of the intrusion alarm system, for example, in particular by placing a glass plate in front of the detector of the passive infrared sensor or by painting the window of the passive infrared sensor in a similar color. A further drawback of the intrusion alarm system known from European patent application No. 0 255 812 is that it is complex and relatively costly, in particular owing to the use of two separate sensors, and because no alarm signal is generated unless both the passive infrared sensor and the radio frequency sensor detect an intruder in the location to be monitored, so that, when one of the sensors does not function at all, or not optimally, no alarm signal is generated. The intrusion alarm system known from U.S. Pat. No. 3,703,718 appears to be rather prone to sabotage in practice.
The object of the invention is to provide a simple and inexpensive intrusion alarm system which makes it possible to detect sabotage to the passive sensor thereof.